Expressur

Expressur does some basic math.

The real reason I built Expressur is to be a meaningful but straightforward set of code that can be ported to almost any other language so that the languages can be compared. It does this by taking a normal problem, arithmetic, and using string manipulation, iteration, and primitive operations.

It can also calculate the results of a set of formula, including formula that rely on the results of other formula. For example, this test from EvaluatorTests.cs shows this "extra indirection" where one formula relies on the results from two other formula, including a formula that in turn relies on other formula.

[Fact]
public void EvaluateExpressions_Evaluates_Correctly_With_Context()
{
    IDictionary<string, string> formula = new Dictionary<string, string> 
    { 
        { "cplusabplusb", "c + aplusb" }, 
        { "aplusb", "a + b" }, 
        { "extraindirection", "aplusb/cplusabplusb" } 
    };

    IDictionary<string, decimal?> context = new Dictionary<string, decimal?> 
    { 
        {"a", 1 }, 
        { "b", 2 }, 
        { "c", 4 } 
    };

    var results = (new Evaluator()).EvaluateExpressions(formula, context);

    Assert.Equal(3m, results["aplusb"].Value);
    Assert.Equal(7m, results["cplusabplusb"].Value);
    Assert.Equal(0.429m, results["extraindirection"].Value, 3); 
        // the 3 as the third parameter is the number of decimal places to check.
}

This uses the Shunting Yard Algorithm to convert the expressions into Reverse Polish Notation in order to handle operator precedence. This is a relatively old school technique suitable for handling arithmetic expressions, but won't be a good basis for building a whole programming language.

PseudoGrammar

identifier := [A-Za-z_][A-Za-z_0-9*]

number := ^-?[0-9]\d*(.\d+)?$

token := identifier | number

operator := [*/+-%^=]

expression := [(]expression|token operator expression|token[)]

Expressur handles all numbers as Base-10 decimals. This will meet most end users' expectations for most scenarios.

Operators supported

• "+" - addition (1 + 1 equals 2)
• "-" - subtraction (2 - 2 equals 0)
• "*" - multiplication (3 * 3 equals 9)
• "/" - division (4 / 4 equals 1)
• "%" - remainder (5%2 equals 1)
• "^" - power (6^6 equals 46656)
• "=" - equals (7=7 equals 1 [true], 7=9 equals 0 [false])

Updated to .NET 8

Expressur has been updated to .NET 8. Performance is substantially improved just with the upgrade. See results here.